There is disclosed in WO 97/17157 a face milling cutter employing a double-sided indexable cutting insert having a prismoidal shape with two opposed generally rectangular rake surfaces connected by side surfaces. The cutting insert has a basic “negative” geometry and therefore in order to provide the necessary clearance between the cutting insert and the workpiece, when mounted in a face-mill, the cutting insert is oriented with a negative axial rake angle. However, negative axial rake angles may be disadvantageous, e.g., they have been found to be deficient in cutting efficiency for applications involving difficult to machine materials.
A double-sided indexable cutting insert for a boring tool head is disclosed in U.S. Pat. No. 5,333,972. The insert is provided at each end with a protruding flat island. Each long cutting edge is inclined at an angle of 3° relative to the protruding flat island, defining an “insert axial rake angle”. Rearward of each cutting edge is a descending land surface that merges with an increasing incident angle surface to form a chip breaker groove. Each increasing incident angle surface extends from its associated descending land surface to an adjacent island, at either the top or the bottom of the cutting insert. The cutting insert is left or right handed. It is manufactured to be right-handed and, when flipped around, is left-handed. It will be appreciated that the magnitude of the insert axial rake angle is limited for practical reasons. Any increase in the insert axial rake angle will result in an increase in the “vertical” extent of the increasing incident angle surface (see FIG. 3 of U.S. Pat. No. 5,333,972) that may have an adverse effect on chip development and evacuation.
There is disclosed in WO 96/35536 a double-sided indexable cutting insert which when mounted in a face-mill has a positive axial rake angle, even when the necessary clearance between the cutting insert and the workpiece is provided. This cutting insert presents two peripheral edges for a right-hand face mill and two peripheral edges for a left-hand face mill. In a side view (see FIG. 9) the cutting insert is generally rhomboidal in shape. The main cutting edges 10 of each end surface are parallel (see also FIGS. 7 and 8) to each other and to a centrally located protruding abutment member 12. The cutting insert is retained in an insert pocket with the abutment member of a non-operative end surface and an adjacent non-operative relief surface in abutment with respective support surfaces of the insert pocket. The abutment member of the non-operative end surface and the adjacent non-operative relief surface merge at an acute-angled mounting corner. In order to change the axial rake angle of the operative cutting edge, either the insert pocket has to be rotated, or a cutting insert having a mounting corner with a different mounting angle has to be used. In either case, a different milling cutter has to be used. Moreover, the axial rake and relief angles are interdependent and any change in the axial relief angle results in a corresponding change in the relief angle, which is not always desirable.
U.S. Pat. No. 5,853,267 describes a single-sided “reversed-relief” indexable cutting insert having side surfaces with, adjacent to the cutting edges, upper primary reliefs that are inclined to the insert base at an acute interior angle, that is, the reverse of positive inserts. Cutting inserts having side surfaces which are inclined to the insert base at an obtuse interior angle are known as “positive” inserts. Cutting inserts having side surfaces which are at right angles to the insert base are known as “negative” or “neutral” inserts. A principal advantage of negative inserts is that they can be designed to be double-sided and therefore provide twice as many indexable cutting edges in comparison with positive inserts. There are also known in the art double positive inserts, which also provide twice as many indexable cutting edges in comparison with positive inserts. However, the cutting edges of such inserts may be weaker than those of negative inserts since the cutting wedge angles of double positive inserts are smaller than those of negative inserts.
Cutting inserts of the type described in US '267, that is, single-sided reversed-relief inserts, are advantageous in that the cutting edges of these inserts are generally stronger than in negative or positive inserts. However, the cutting insert disclosed in US '267 is a one-sided beveled relief cutting insert. Moreover, the through bore for receiving a clamping screw opens out to the upper chip surface. This sets a limit on the minimum width (distance between the major cutting edges) of the cutting insert. In addition, the cutting insert disclosed in US '267 is “radially” oriented when seated in a milling cutter. In such cutting inserts, the distance between the cutting edges and the base of the cutting insert defines a minimal dimension of the cutting insert. During a cutting operation, the cutting forces are directed generally along the minor dimension. On the other hand, tangential cutting inserts, also known as on-edge, or lay down, cutting inserts, are oriented in an insert holder in such a manner that during a cutting operation on a workpiece the cutting forces are directed along a major (thicker) dimension of the cutting insert. An advantage of such an arrangement being that the cutting insert can withstand greater cutting forces than when oriented in such a manner that the cutting forces are directed along a minor (thinner) dimension of the cutting insert. The present invention relates to double-sided reversed-relief cutting inserts.
Although it is conceivable to obtain double-sided reversed-relief inserts by grinding primary relief adjacent the cutting edges at both ends of a negative insert, grinding is an additional manufacturing operation that increases time and production costs of the cutting inserts.
It is an object of the present invention, to provide a double-sided tangential reversed-relief cutting insert which is preferably pressed or molded into its final shape, having a through bore for receiving a clamping screw which does not pass through the chip surfaces. Since double-sided inserts do not have a base opposite the chip surface, the angle of the primary relief is measured with respect to a median plane of the cutting insert.